1. Field of the Invention
The present invention relates to dry silver photothermographic imaging materials and to toners used therein.
2. Prior Art
Photosensitive, heat-developable, dry silver sheet materials, as described for example in U.S. Pat. Nos. 3,457,075 and 3,839,049, contain a photosensitive silver halide catalyst-forming means in catalytic proximity with a heat sensitive combination of a light stable organic silver compound and a reducing agent therefor. When struck by light, the silver halide catalyst-forming means produces silver nuclei which serve to catalyze the reduction of the organic silver compound, e.g., silver behenate, by the reducing agent at elevated temperatures. To improve the image density and color it has been found desirable to include toners in the sheet construction. Phthalimide has been known as such a toner.
In U.S. Pat. No. 3,080,254, phthalazinone is described as a toner for dry silver sheets. Phthalazinone, however, has been found to cause slight dusting that becomes noticeable during heat development of large numbers of the exposed sheets. The dusting problem can be avoided by using as toner a combination of an imidazole and phthalic acid, naphthalene-2,3-dicarboxylic acid of phthalamic acid, as described in U.S. Pat. No. 3,847,612, if a hindered phenolic reducing agent for silver ion is also present in the sheet. The resulting dry silver sheets tend to have lower optical speed and have lower light stability after development than sheets containing phthalazinone toner.
U.S. Pat. No. 4,123,282 discloses the use of a combination of phthalazine and aromatic acids as a toner system for providing dense, dark images in dry silver photothermographic emulsions. The aromatic acids tend to react with developer, dyes and other ingredients in the emulsion thereby reducing the shelf-stability of the emulsions.